The original Bitcoin software by Satoshi Nakamoto was released under the MIT license.
Most client software, derived or "from scratch", also use open source licensing.

Bitcoin is the first successful implementation of a distributed crypto-currency, described in part in 1998 by Wei Dai on the cypherpunks mailing list. Building upon the notion that money is any object, or any sort of record, accepted as payment for goods and services and repayment of debts in a given country or socio-economic context, Bitcoin is designed around the idea of using cryptography to control the creation and transfer of money, rather than relying on central authorities.

Bitcoins have all the desirable properties of a money-like good. They are portable, durable, divisible, recognizable, fungible, scarce and difficult to counterfeit.

Why?

Bitcoin is P2P electronic cash that is valuable over legacy systems because of the monetary autonomy it brings to its users. Bitcoin seeks to address the root problem with conventional currency: all the trust that's required to make it work -- Not that justified trust is a bad thing, but trust makes systems brittle, opaque, and costly to operate. Trust failures result in systemic collapses, trust curation creates inequality and monopoly lock-in, and naturally arising trust choke-points can be abused to deny access to due process. Through the use of cryptographic proof, decentralized networks and open source software Bitcoin minimizes and replaces these trust costs.

Are in your direct possession with no counterparty risk. If you keep the private key of a bitcoin secret and the transaction has enough confirmations, then nobody can take them from you no matter for what reason, no matter how good the excuse, no matter what.

FAQ

A. Bitcoin is a peer-to-peer currency. Peer-to-peer means that no central authority issues new money or tracks transactions. These tasks are managed collectively by the network.

Q. How does Bitcoin work?

A. Bitcoin uses public-key cryptography, peer-to-peer networking, and proof-of-work to process and verify payments. Bitcoins are sent (or signed over) from one address to another with each user potentially having many, many addresses. Each payment transaction is broadcast to the network and included in the blockchain so that the included bitcoins cannot be spent twice. After an hour or two, each transaction is locked in time by the massive amount of processing power that continues to extend the blockchain. Using these techniques, Bitcoin provides a fast and extremely reliable payment network that anyone can use.